As a technology keeping up with changing conditions of wireless lines, an adaptive modulation method has been conventionally proposed. The adaptive modulation method discriminates conditions of wireless lines so as to perform communication according to a modulation scheme with a low multi-value modulation number when conditions are poor or to perform communication according to a modulation scheme with a high multi-value modulation number when conditions are good. Specifically, a receiver estimates a condition of a wireless line based on a received signal so as to determine its modulation scheme, thus feeding back the determined modulation scheme to a transmitter. The transmitter adopts the modulation scheme, which is fed back from the receiver, so as to carry out its transmitting process. This adaptive modulation method has been widely adopted in fixed microwave radio transmission devices and radio base stations for use in mobile communications.
As a determination method of a modulation scheme in an adaptive modulation method, technologies with high and precise traceability with changing conditions of wireless lines, such as technologies disclosed in PLT 1 and PLT 2, have been conventionally proposed.